TurnCustomizer - an interface for modifying stun messages sent by TurnPort

This patch adds an interface that allows modification of stun messages
sent by TurnPort. A user can inject a TurnCustomizer on the RTCConfig
and the TurnCustomizer will be invoked by TurnPort before sending
message. This allows user to e.g add custom attributes as described
in rtf5389.

BUG=webrtc:8313

Change-Id: I6f4333e9f8ff7fd20f32677be19285f15e1180b6
Reviewed-on: https://webrtc-review.googlesource.com/7618
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20233}

p2p/base/stunrequest.h

@@ -105,6 +105,9 @@ class StunRequest : public rtc::MessageHandler {
   // Returns a const pointer to |msg_|.
   const StunMessage* msg() const;
 
+  // Returns a mutable pointer to |msg_|.
+  StunMessage* mutable_msg();
+
   // Time elapsed since last send (in ms)
   int Elapsed() const;